Concern over the depletion and environmental impact of fossil fuels has stimulated strong interest in the development of alternative energy sources. Significant investments in areas such as batteries, fuel cells, hydrogen production and storage, biomass, wind power, algae, and solar energy have been made as society seeks to develop new ways of creating and storing energy in an economically competitive and environmentally benign fashion. The ultimate objective is to minimize society's reliance on fossil fuels to minimize greenhouse gas production and to do so in an economically competitive way.
A number of experts have concluded that to avoid the serious consequences of global warming, it is necessary to maintain CO2 at levels of 550 ppm or less. To meet this target, based on current projections of world energy usage, the world will need 17 TW of carbon-free energy by the year 2050 and 33 TW by the year 2100. The estimated contribution of various carbon-free sources toward the year 2050 goal are summarized below:
Projected EnergySourceSupply (TW)Wind2-4Tidal2Hydro1.6Biofuels5-7Geothermal2-4Solar600Based on the expected supply of energy from the available carbon-free sources, it is apparent that solar energy is the best solution for reducing greenhouse emissions and alleviating the effects of global climate change.
Unless solar energy becomes cost competitive with fossil fuels, however, society will lack the motivation to eliminate its dependence on fossil fuels and will refrain from adopting solar energy on the scale necessary to meaningfully address global warming. As a result, current efforts in manufacturing are directed at reducing the unit cost (cost per kilowatt-hour) of energy produced by photovoltaic materials and products. The general strategies for decreasing the unit cost of energy from photovoltaic products are improving photovoltaic efficiency and reducing process costs.
Crystalline silicon is currently the dominant photovoltaic material because of its wide availability in bulk form and mature technology base. Crystalline silicon, however, suffers from a number of drawbacks as a photovoltaic material. Crystalline silicon, for example, possesses weak absorption of solar energy because it is an indirect gap material. As a result, photovoltaic modules made from crystalline silicon are thick, rigid and not amenable to lightweight, thin film products. The deficiencies of crystalline silicon create opportunities for new technologies. In order to be commercially viable, however, new solar technologies must provide a value proposition (cost in relation to benefit) that is superior to that of crystalline silicon.
Materials that absorb wavelengths of the solar spectrum more strongly than crystalline silicon are under active development for photovoltaic products. Representative materials include CdS, CdSe, CdTe, ZnTe, CIS (Cu—In—Se and related alloys), CIGS (Cu—In—Ga—Se and related alloys), organic materials (including organic dyes), and TiO2. These materials offer the prospect of reduced material costs because their high solar absorption efficiency permits photovoltaic operation with thin films, thus reducing the volume of material needed to manufacture devices and expanding the range of applications.
Amorphous silicon (and hydrogenated and/or fluorinated forms thereof) is another attractive photovoltaic material for lightweight, efficient, and flexible thin-film photovoltaic products. Stanford R. Ovshinsky was among the first to recognize the advantages of amorphous silicon (as well as amorphous germanium, amorphous alloys of silicon and germanium, including doped, hydrogenated and fluorinated versions thereof) as a photovoltaic material. S. R. Ovshinsky also recognized the underlying physical properties and practical benefits of the nanocrystalline, microcrystalline, and intermediate range order forms of silicon, germanium, silicon-germanium alloys and related materials. For representative contributions of S. R. Ovshinsky in the area of photovoltaic materials see U.S. Pat. No. 4,217,374 (describing suitability of amorphous silicon and related materials as the active material in several semiconducting devices); U.S. Pat. No. 4,226,898 (demonstration of solar cells having multiple layers, including n- and p-doped); and U.S. Pat. No. 5,103,284 (deposition of nanocrystalline silicon and demonstration of advantages thereof); as well as his article entitled “The material basis of efficiency and stability in amorphous photovoltaics” (Solar Energy Materials and Solar Cells, vol. 32, p. 443-449 (1994)).
In addition to the identification of photovoltaic materials with stronger solar absorption, new technologies seek to lower unit cost of solar energy through process efficiency. Process efficiency can be improved by increasing process speed (throughput), adopting materials amenable to high-speed processing, simplifying process steps, and utilizing continuous (as opposed to batch) processing for as many manufacturing steps as possible.
The instant inventor, S. R. Ovshinsky, has pioneered the automated and continuous manufacturing techniques needed to produce thin film, flexible large-area solar panels based on amorphous, nanocrystalline, microcrystalline, polycrystalline or composite materials. Although his work has emphasized the silicon and germanium systems, the manufacturing techniques that he has developed are universal to all material systems. Representative contributions of S. R. Ovshinsky to the field of high speed thin film manufacturing are included in U.S. Pat. No. 4,400,409 (describing a continuous manufacturing process for making thin film photovoltaic films and devices); U.S. Pat. No. 4,410,588 (describing an apparatus for the continuous manufacturing of thin film photovoltaic solar cells); U.S. Pat. No. 4,438,723 (describing an apparatus having multiple deposition chambers for the continuous manufacturing of multilayer photovoltaic devices); and U.S. Pat. No. 5,324,553 (microwave deposition of thin film photovoltaic materials).
In addition to the intrinsic properties of the active photovoltaic material, the characteristics of the surrounding layers in the device structure are important for optimizing photovoltaic conversion efficiency. Photogenerated charge carriers that avoid recombination are ultimately transported to the surface of the active photovoltaic material. In a typical device structure, the active photovoltaic material is positioned between two electrodes (that interconnect the photovoltaic device to an external circuit) and a voltage applied across the electrodes (provided by an external source or load, or present as a built-in voltage) drives the transport of photogenerated charge carriers to the surface of the photovoltaic material. At the surface, the carriers enter the electrodes and are delivered to an external load.
High photovoltaic efficiency requires efficient transfer of photogenerated carriers from the photovoltaic material to the electrodes and efficient transport of the carriers through the electrodes to an interconnected external load. Efficient transfer can be achieved by insuring a high quality interface between the photovoltaic material and electrodes and forming ohmic contact. When the electrode materials are metals, transport through the electrodes is highly efficient and leads to no appreciable decrease in photovoltaic conversion efficiency. In many common photovoltaic device structures, however, it is not feasible to utilize metals for both contacts surrounding the photovoltaic material. In order to achieve high speed continuous production, for example, it is necessary to use a substrate that is capable of being quickly transported along a manufacturing line. The substrate must be able to withstand mechanical handling during production without fracturing.
The substrate requirements in high speed continuous manufacturing are best met by steel or other metal substrates. Steel is sufficiently durable to maintain its structural integrity during mechanical transport and is not subject to fracture under the tensile stresses produced by conveyance. Since steel retains its durability when thin (unlike more fragile substrates like glass), steel substrates provide a lightweight solution for high speed production. Steel also has high electrical conductivity and is an effective electrode material.
The drawback of steel as a substrate for photovoltaic applications is its opacity. When used as a substrate, steel blocks incident light and prevents it from interacting with the active material of a photovoltaic device. As a result, the opposing electrode of the device must be transparent to enable incident light to excite the active photovoltaic material. A number of transparent electrode materials are known, including the widely used ITO (indium tin oxide) series of transparent conductive oxides. Although functional, the known transparent conductive materials are much less conductive and have much higher sheet resistivities than metal electrode materials. The high sheet resistivity introduces series resistive (I-R) losses through the transparent electrode and degrades the current supplied by the photovoltaic device. The overall conversion efficiency suffers as a result.
The lower conductivity of transparent electrodes becomes especially disadvantageous in large-area photovoltaic devices. In order for current to reach the external leads, it is necessary for the photogenerated charge carriers to exit the photovoltaic material, enter the transparent electrode, and migrate in a direction parallel to the surface of the active photovoltaic material to the external leads that contact the perimeter of the transparent electrode. As the area of the active photovoltaic material increases, the average distance that charge carriers must travel from the interior of the transparent electrode to the perimeter increases. The increased distance increases the resistance of charge carriers that flow from the photovoltaic material to the external leads and results in a lower current and higher resistive (I-R) power loss.
To facilitate carrier transport in transparent electrodes, it is common to place a mesh of metal wires on the top (light-incident) surface. The mesh is often referred to as a current collector or current collection grid. The metal wires of the current collection grid have high conductivity and are interconnected to the leads placed in contact with the perimeter of the transparent electrode. The grid expedites transport of charge carriers to the external leads at the perimeter of the transparent electrode by providing alternative, high conductivity pathways that have the effect of lowering series resistance. In photovoltaic devices that utilize transparent electrodes, the transparent electrode is a thin film that has a thickness normal to the active photovoltaic material that is much shorter than the lateral dimension of the transparent electrode. As a result, the presence of a current collection grid on the top surface reduces the distance that charge carriers need to migrate before reaching a metallic conductor. Instead of needing to migrate over a long lateral distance to reach the metallic conductors at the perimeter, charge carriers that enter the transparent electrode need only migrate over the shorter thickness dimension of the transparent electrode to reach the metallic conductors of the current collection grid. The resistive losses of charge carriers within the transparent conductor are reduced accordingly. A shorter migration distance also reduces the residence time of charge carriers in the transparent conductor, which decreases the likelihood that charge carriers will recombine with defects. As a result, higher currents and more efficient recovery of the photogenerated charge carriers produced in the active photovoltaic material are obtained.
Since the current collector is opaque and is placed on the light-incident side of the transparent electrode, it has the effect of reducing the amount of light that reaches the active photovoltaic material. The reduction in the amount of incident light may be referred to as a “shadowing effect”. In order to realize a net benefit from the current collector, the wires that comprise the grid must be thick enough to materially facilitate conduction of charge carriers from the transparent electrode, but not so thick as to appreciably diminish the amount of light that excites the active photovoltaic material. In practice, it is found that a fine mesh of wires provides the optimum benefit.
The disadvantage of current collectors from a manufacturing perspective is that they are difficult to integrate with a photovoltaic device. The process of positioning and aligning individual wires or a pre-fabricated mesh of thin wires is time-consuming, labor intensive and not readily automated. As a result, both the cost of production and time of manufacturing are increased. A need exists in the art for a simpler, more effective method for integrating a current collection grid with a transparent conductor to boost the performance of photovoltaic devices formed on opaque substrates. The method should be inexpensive, fast and easy to implement in a high speed manufacturing process.